Grid excitation



Feb. 12, 1957 G. R. MOHR 2,781,449

GRID EXCITATION Filed Aug. 20. 1954 9 Fig.|.

Prior Art Prior Ar? WITNESSES] INVENTOR fiz/wr Glenn R.Mohr.

nitd rates acn 2,781,449 Patented Feb. 12, 1957 G EXCITATION Application August 20, 1954, Serial No. 451,087

Claims. (Cl. 250-36) The present invention relates to power supply apparatus, and, more particularly, to the feedback circuit in power supply apparatus for high frequency heating of bodies.

In the high frequency induction heating of a load, such as metallic workpieces, best results are obtained when the voltage across the load coil and the current in the load coil are at maximum value. When the load is supplied by an oscillator having the load coil in series with the tank coil of the oscillator, an increase in the inductance of the load will necessitate the shorting out of a few turns of the tank coil in order to maintain the frequency of operation of the oscillator supplying the heating kva. to the load. It is necessary to maintain this frequency of operation in order to keep the current in the tank circuit at a maximum. In several of the oscillators employed in high frequency induction apparatus, which are known in the art and are utilized at present, the necessary shorting out of the turns in the tank coil results in the undesirable effect of a considerable decrease in the tank circuit kva. which produces the heating of the load.

It is therefore an object of my invention to provide an improved oscillator for use in high frequency induction heating apparatus.

It is another object of my invention to provide an improved oscillator in which a novel division of the inductor of the tank circuit into two portions is employed with minimum mutual induction effected between the two portions of the inductor.

It is an additional object to provide an improved oscillator in which the tank inductance may be in the form of two coils with the grid excitation being obtained inductively from one coil without being appreciably effected by any electrical changes occurring in the second coil, such as shorting out of turns in the second coil to maintain a constant frequency of operation for the oscillator.

These and other objects of the invention are effected as will be apparent from the following description taken in accordance with the accompanying drawings, which form a part of this application and in which like numerals, are employed to designate like parts throughout the same:

Figure l is a schematic diagram of an oscillator circuit for use in high frequency induction heating apparatus, which circuit is well known in the art;

Fig. 2 is a schematic diagram of the circuit of another oscillator which is employed at present in high frequency induction heating apparatus; and

Fig. 3 is a schematic diagram of a novel oscillator circuit for high frequency induction heating apparatus, in accordance with my invention.

In Fig. 1 is shown an oscillator circuit which is well as 360 kc. to SOOkc.

known in the art in which the coil 6 for eifecting the excitation of the grid 32 of the electron dischrage device 28 is inductively coupled with the tank inductor a. As the inductance of the workpiece positioned in load coil 12 increases, either by the properties of the workpiece or by the size of the load eoil 12, it. becomes necessary to short out turns in inductor 8 to maintain the frequency of operation of the oscillator. The turns of the inductor 8 form an autotransformer when the turns are shorted, with the shorted turns forming a shorted secondary. The flux in the shorted 'turns will be in such a direction as to reduce the flux in the tank inductor and therefore reduce the excitation of grid 28. It has been found that 15% of the total kva. of the tank circuit is required for grid excitation when the turns of the inductor 8 are not shorted but when the latter turns are shorted, 40% of the total tank circuit kva. is re quired for grid excitation.

In Fig. 2 is shown another oscillator circuit in which the coil 6 for effecting the excitation of the grid 32 of the electron discharge device 28 is inductively coupled with an auxiliary inductor 2%, with the inductor 20 being connected in shunt with the tank circuit 11 of the oscillator. The serious loss in kva. dueto the shorting of turns in tank inductor 8 is not realized in this circuit. However, oscillators in high frequency induction heating apparatus operate over a range of frequencies, such If the grid circuit of the oscillar. tor as shown in Fig. 2 is designed for minimum excita:

tion, approximately 15% of the total tank circuit kva. is required for grid excitation at 500 kc., and at 300 kc., the grid excitation is increased to 25% of the total tank circuit kva. Also, since the total tank circuit vol age is constant because of the D. C. plate voltage supplied, a decrease in frequency from 500 kc. to 360 kc. will increase the current in the shunt coil 20 but decrease the current in the tank circuit capacitor 10. Therefore, the proportion of. current in the shunt coil 20 will be increased. The load coil impedance must also increase, otherwise there could not be a decrease infrequency. Hence, due to the lower frequency of 300 kc., the total kva. of the tank circuit has also decreased to three-fifths of its original value. Consequently, the percentage of total tank circuit kva. necessary for grid excitation has increased from 15% at 500 kc. to 41.7% at 300 kc.

In Fig. 3 is shown a schematic diagram cl :1 novel oscillator, in accordance with my invention, in which the disadvantages of the above two oscillators. are effectively reduced. in the oscillator shown in thi figure, a triode 2%} is provided with a plate circuit including a D. C. source connected to the plate 3% of the triode and a tank circuit 13. The tank circuit 13 includes a tuning condenser 14) and a parallel branch comprising .a serially connected first tank inductor coil 24, asecond tank inductor coil 26 and a load coil 12. The number of turns in the first tank inductor coil 24- may be in the order of about 15% of the total inductance of the tank circuit.

the latter two coils 24 and 26 are substantially perpendicular to each other to provide minimum coupling between,

the coil 24 and the coil 25 when the oscillator is operatanode 30 of the triode 28 through the blocking capacitor 14. The resistor 38 and the parallel by-pass capacitor 40 are connected between the grounded end of the tank circuit 13 and the cathode 36 of triode 28 to provide suitable cathode bias for the triode. To maintain high frequency oscillations a grid circuit is provided with a grid excitation or feedback coil 6 connected in serie with the grid $2 and the parallel grid leak resistor 18 and capacitor 16. The longitudinal axis of the coil 6 should be positioned substantially parallel to the longitudinal axis of the first tank inductor coil 24 so that sufficient electrical coupling will be effected between the latter two coils to provide excitation for the grid 32, while the axis of the coil 6 should also be positioned substantially perpendicular to the longitudinal :axis of the second inductor coil 26 in order that minimum electrical coupling will be effected between coil 6 and second inductor 26. V

In accordance with my invention as shown in Fig. 3, the grid excitation coil 6 may be designed to receive inductively a small percentage of the total tank circuit kva., such as when operated at the minimum value of frequency of the frequency range of the oscillator. As the load inductance is increased, some of the turn of the second tank coil 26 may be shorted to maintain the frequency of the oscillator. Since the turns of the grid excitation coil are physically positioned at substantially 90 to the second tank coil 26, minimum coupling is effected between the two coils. Hence, there i no elfective reduction in grid excitation when some of the turns in the second tank coil 26 are shorted. Also, since some of the total inductance of the tank circuit 13 is supplied by the first tank coil 24, the number of turns in the second tank coil 26 of Fig. 3 will necessarily be less than the number of turns in the single tank coil8 in both of the circuits shown in Figs. 1 and 2, for operation of all three circuits under like conditions of load and frequency. Hence, when a number of turns of the second tank inductor 26 in Fig. 3 must'be shorted to maintain the frequency of operation of the oscillator, the turns ratio of the autotransformer will be less than would be the case in the circuits in Figs. 1 and 2. Consequently, the losses in the tank circuit 13 of Fig. 3 would be less than the losses in the tank circuits 9 and 11 in Figs. 1 and 2 under like operating conditions.

If the grid excitation coil 6 i designed to receive inductively a small percentage of the total tank circuit kva., such as 15%, when operated at the minimum value of frequency, then as the value of frequency of operation of the oscillator is increased the percentage of the total tank circuit kva., received by the grid circuit will be increased. However, the magnitude of kva. required in the load circuit will decrease a proportional amount. Therefore, no substantial change is realized in the heating of the load when the frequency of operation of the oscillator is changed.

While I have shown the use of my invention in a single embodiment, it will be obvious to those skilled in the art that it is not so limited but is susceptible to Various changes and modifications without departing from the spirit thereof. For example, a single coil construction having a first portion of windings and a second portion of windings, with the longitudinal axis of the first portion being so positioned that it is substantially perpendicular to the axis of the second'portion, may be substituted for the tank inductor coils 24 and 26 in the oscillator shown in Fig. 3. .The novel construction of the tank inductors in combination with a grid excitation coil may be used to supply feedback for grid excitation in various types of power. supplies, whether the feedback is sufiicient to produce oscillations in an oscillator or employed to effect either regenerative or degenerative feedback in feedback amplifiers. The output .of the oscillator need not only be supplied to a load coil 12 as shown, but may be sup- 1. In power supply apparatus including an electron discharge device having an anode, a grid and a cathode, the combination of a tank circuit and an excitation coil, said tank circuit including a first helical coil and a second helical coil, with a capacitive member connected in parallel with the latter coils, a load coil, said load coil being connected in a series arrangement with said first coil and said second coil, said tank circuit being connected in the plate circuit of said electron discharge device, with said first coil and said second coil being so positioned that their respective axes are substantially perpendicular to each other, said excitation coil being connected to the grid of said electron discharge device and being positioned for electrical coupling with said first coil thereby to provide excitation for said grid.

2. In power supply apparatus including an electron discharge device having an anode, a cathode and a control grid, the combination of a tank capacitor and a helical tank inductor in a tank circuit arrangement, with said tank circuit being connected in the plate circuit of said electron discharge device, a load coil, said load coil being serially connected to said' tank capacitor and said tank inductor, said tank inductor having a first portion and a second portion, with the axis of said first portion being positioned substantially perpendicular to the axis of said second portion, an excitation coil, said excitation coil being connected to said grid and being positioned with its axis in parallel with said first portion for electrical coupling between the first portion of said tank inductor and the excitation coil thereby providing excitation of the grid of said electron discharge device.

3. In power supply apparatus for supplying a load, the combination'of an electron discharge device for controlling the output of said apparatus, a tank circuit for determining the output frequency of said apparatus and a means forsupplying feedback to said electron discharge device, said electron discharge device comprising an anode and a control grid, said tank circuit comprising a tank capacitor and a tank inductor connected in parallel, with means provided for connecting said load in series with said capacitor and said inductor, said tank inductor having a first'portion and a second portion, with said first portion being so positioned that substantially minimum coupling will be effected between said first portion and said second portion, said means for supplying feedback to said electron discharge device comprising an excitation coil connected to said grid of the device, with said excitation coil being electrically coupled to said first portion of said tank inductor, thereby effecting excitation of said grid.

4. In power supply apparatus for supplying a load, with said apparatus including an electron discharge device having an anode and a control grid, the combination of a tank circuit, and a means for providing excitation of said grid, said tank circuit connected in the plate circuit of said electron discharge device, with said tank circuit comprising a first inductor and a second inductor and a capacitor in parallel with said inductors, means provided for connecting said load in series with said inductors and said capacitor, said first inductor being positioned transversely with respect to the second inductor for minimum coupling plied to any suitable load that may be substituted for the load coil in the circuit, in accordance with my between the latter two inductors, said means for excitation of said grid comprising a coil connected to said grid, with said coil being positioned adjacent to said first inductor and being electrically coupled with the latter inductor, thereby providing an excitation potential for said grid.

5. In power supply apparatus for supplying a load, the combination of an electron discharge device for controlling the output of said apparatus, a tank circuit for determining the output frequency of said apparatus and a first means for supplying feedback to said electron discharge device, said electron device comprising an anode and a control grid, said tank circuit including serially connected first and second tank inductors and a parallel tank capaci tor, with the axis of said first tank inductor being positioned substantially perpendicular with respect to the axis of said second tank inductor so that minimum coupling will be effected between the latter two inductors, a second means provided for connecting said load in series with said capacitor and said inductors, said first means for supplying feedback to said electron discharge device comprising an excitation coil connected to said grid of the device,

with said excitation coil being electrically coupled to said first tank inductor, thereby providing an excitation potential on said grid.

References Cited in the file of this patent UNITED STATES PATENTS 2,341,120 Rudd et a1 Feb. 8, 1944 

